Absorption Refrigeration
A gas powered refrigerator uses the heat of the gas pilot to drive the cooling process by creating liquid ammonia in a hydrogen environment. Just as your sweat helps cool you by evaporating on a hot day, the ammonia cools by evaporating.
The process is a little complex, but the link below seems to explain it well. In a nutshell:
Liquid ammonia is in an evaporator tube with hydrogen gas. The hydrogen helps create a condition by which the ammonia boils off into a vapor (evaporates), cooling the tube in the process. The resulting ammonia gas - hydrogen gas mixture then enters a tube (absorber) with dripping water. The ammonia is absorbed into the water, while the hydrogen is not absorbed. The water with the ammonia absorbed into it flows to the bottom of the coil system. There, an external heat sources (like a gas flame) boils the water-ammonia mixture so that only the ammonia bubbles out (generator). The bubbles contain water, so the tubes have intricacies that cause the bubbles to burst (separator). The water is separated and enters the previously mentioned "dripping" part of the cycle, while the ammonia gas - which now has no hydrogen - goes into an external set of tubes (heat exchanger) that use room air to cool the hot ammonia gas so that it liquifies (condenser). It then enters the evaporator again, where hydrogen gas is present, and the process starts over again.
So yes, it seems counter-intuitive that a heat source can be used to cool a chamber.
But the heat is only used to generate ammonia vapor from a water-ammonia mixture. The rest of the process uses a maze to separate the water from the ammonia vapor, and an external heat exchanger to cool the hot ammonia vapor to room temperature and condense the ammonia vapor into liquid ammonia. Then, in the presence of hydrogen gas to lower the boiling point of the liquid ammonia, the ammonia evaporates and cools the chamber to be refrigerated.
The process uses several interesting physical-chemical processes: evaporation (which does the cooling by evaporating liquid ammonia in a hydrogen gas environment), absorption (which separates the ammonia vapor from the hydrogen gas by absorbing the ammonia into water), generation (which uses the gas flame to generate ammonia-water vapor from the water-ammonia mixture), separation (which separates the ammonia vapor from the water bubbles), condensation (which uses a heat exhanger to cool the hot ammonia vapor to room temperature and turn it into liquid ammonia). The whole process has no moving parts, is quiet, and can last a long time. It's kind of like a closed loop still with various ingenious ways to isolate the heat required to generate ammonia vapor from a water-ammonia mixture, from the heat absorbed by liquid ammonia evaporating in a hydrogen gas environment. It also manages to keep water, hydrogen, and ammonia separated or combined at the right times in the right places.
According to the linked article, heat from any source can be used to drive the generation part of the process, including solar thermal heat. Even electricity can be used, although that seems like it sort of defeats the off-grid advantage of an absorption system and it's also more costly than gas or solar.
en.wikipedia.org
